Put bin/traceur-runtime.js in npm

The npm package now contains bin/traceur-runtime.js instead of src/runtime/runtime.js. Previously, users could use the package to compile multiple modules into one ES5 file, but didn't have the necessary runtime code from src/runtime/modules.js to use the result.

The file is created with a npm prepublish script. npm runs this script during `npm publish`, `npm pack` and `npm install --local`. Traceur's Makefile uses `npm install --local`, so this is a bit tricky.

Fixes #587.

Review URL: https://github.com/google/traceur-compiler/pull/592

Closes #592, closes #587.